Various methods and systems are known in the art for tracking the coordinates of objects involved in medical procedures. Some of these systems use magnetic field measurements. For example, U.S. Pat. Nos. 5,391,199 and 5,443,489, whose disclosures are incorporated herein by reference, describe systems in which the coordinates of an intrabody probe are determined using one or more field transducers. Such systems are used for generating location information regarding a medical probe or catheter. A sensor, such as a coil, is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations.
Additional methods and systems that relate to magnetic position tracking are also described, for example, in PCT Patent Publication WO 96/05768, U.S. Pat. Nos. 6,690,963, 6,239,724, 6,618,612 and 6,332,089, and U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. These publications describe methods and systems that track the position of intrabody objects such as cardiac catheters, orthopedic implants and medical tools used in different medical procedures.
Some position tracking systems, including some of the systems described in the above-mentioned references, use alternating-current (AC) magnetic fields. Other position tracking systems use direct-current (DC) fields. For example, U.S. Pat. No. 4,945,305, whose disclosure is incorporated herein by reference, describes a system for measuring the position of receiving antennae with respect to transmitting antennae, utilizing pulsed DC magnetic signals. The transmitting antennae are driven one at a time by a pulsed, direct current signal. The receiving antennae measure the transmitted magnetic fields and the earth's magnetic field. A computer converts the received signals into location and orientation outputs.
U.S. Pat. No. 5,453,686, whose disclosure is incorporated herein by reference, describes a system that generates a plurality of electromagnetic fields by applying time-division multiplexed pulsed DC signals to a plurality of field generating elements. The fields are sensed by remote sensors so as to detect the rate-of-change of each of the generated electromagnetic fields. The outputs of the remote sensors are integrated in order to establish the steady state components of the generated electromagnetic fields. The steady state components are resolved into the remote object's position and orientation.